The present invention relates to nitride semiconductor structures and devices and to processes for making the same.
Nitride semiconductors such as gallium nitride and related semiconductors are widely regarded as desirable wide bandgap compound semiconductors. These materials have been adopted in optoelectronic devices such as light-emitting diodes (“LEDs”), laser diodes and photodiodes, and have also been employed in non-optical electronic devices such as field effect transistors (“FETs”) and field emitters. In optoelectronic devices, the wide bandgap of the material allows for emission or absorption of light in the visible-toultraviolet range. In electronic devices, gallium nitride and related materials provide high electron mobility and allow for operation at very high signal frequencies.
Nitride semiconductors typically are formed by epitaxial growth on a substrate. In an epitaxial growth process, the constituents of the semiconductor film to be formed are deposited on a crystalline substrate so that the deposited semiconductor material has a crystal structure patterned on the crystal structure of the substrate. Various epitaxial growth processes use different techniques for delivering the materials to the surface of the substrate. For example, in reactive sputtering, the metallic constituent of the semiconductor as, for example, gallium, aluminum or indium, is dislodged from a metallic sputtering target in proximity to the substrate in an atmosphere which includes nitrogen. In a process known as metal organic chemical vapor deposition (MOCVD), the substrate is exposed to an atmosphere containing organic compounds of the metals and a reactive, nitrogen-containing gas, most commonly ammonia, while the substrate is at an elevated temperature, typically on the order of 700–1100° C. Under these conditions, the compounds decompose, leaving the metal nitride semiconductor as a thin film of crystalline material on the surface. After growth of the film, the substrate and grown film are cooled and further processed to form the finished devices.
To provide a high quality nitride semiconductor film, with relatively few crystal defects, the substrate used for crystal growth should ideally have a lattice spacing (spacing between adjacent atoms in its crystal lattice) equal to that of the nitride semiconductor to be grown. If the lattice spacing of the substrate is substantially different than that of the grown film, the grown film will have defects such as dislocations in the crystal lattice. Also, the substrate should have a coefficient of thermal expansion equal to or greater than that of the nitride semiconductor to be grown, so that when the substrate and nitride semiconductor are cooled to room temperature after growth, the substrate contracts to a greater degree than the film, placing the film in compression. If the coefficient of thermal expansion of the substrate is substantially smaller than that of the grown film, the film will tend to contract more than the substrate, placing the film in tension when the film and substrate are cooled. This can cause cracks in the film.
Gallium nitride based semiconductors are most commonly grown on crystalline sapphire wafers. Satisfactory results can be achieved on sapphire, despite a relatively large lattice mismatch between sapphire and gallium nitride. Silicon carbide, in theory, is a more desirable material for growth of high-quality gallium nitride, inasmuch as it has a smaller lattice mismatch. Moreover, silicon carbide has higher thermal conductivity than sapphire, which aids in dissipating heat from the finished device. However, high-quality, crystalline silicon carbide wafers are very expensive and, at the present time, are not available in large sizes greater than about 100 mm (4 inches) in diameter.
High-quality silicon substrates are widely available at reasonable cost. However, the lattice spacing of silicon is not well matched to that of gallium nitride. Moreover, silicon has a lower coefficient of thermal expansion than gallium nitride, so that gallium nitride films grown on silicon tend to crack when the film and substrate are cooled to room temperature. Moreover, silicon substrates are relatively poor electrical insulators. Where the deposited nitride semiconductor is used in certain electronic devices, such as FET's, the substrate causes significant electrical losses in the device and limits the performance of the device. For all of these reasons, silicon has not been widely adopted as a substrate for growing nitride semiconductors.
Various proposals have been advanced to compensate for the lattice mismatch and thermal expansion mismatch between the nitride semiconductors and silicon. For example, Nitronics, International Publication No. WO 02/48434, suggests using a “compositionally graded transition layer” formed on a silicon substrate and depositing a gallium nitride material over the transitionally graded layer. The transition layer may contain aluminum indium gallium nitride, indium gallium nitride or aluminum gallium nitride, with proportions of aluminum, indium and gallium varying from a back surface adjacent the substrate to a front surface upon which the semiconductor is to be grown. The compositionally graded layer may include a “superlattice,” i.e., a crystalline structure having a periodic variation in composition as, for example, different amounts of aluminum, indium and gallium.
Another approach taught in Feltin et al., “Stress Control In GaN Grown On Silicon (111) By Metal Organic Vapor Phase Epitaxy,” Applied Physics Letters, Vol. 79, No. 20, pp. 3230–3232 (Nov. 12, 2001), utilizes an aluminum nitride buffer layer in direct contact with the silicon substrate. A layer of gallium nitride is deposited over the aluminum nitride buffer layer, followed by a superlattice including alternating layers of aluminum nitride and gallium nitride, followed by further gallium nitride layers and superlattices and, finally, by a layer of gallium nitride at the top of the structure which constitutes the active semiconductor layer to be grown. According to the Feltin et al. article, this approach yields a high-quality, active layer.
Despite these and other efforts in the prior art, however, it has been difficult to grow high-quality gallium nitride-based semiconductors on silicon substrates. Moreover, devices such as FETs fabricated from gallium nitride-based semiconductors on silicon substrates have suffered from performance problems caused by the silicon substrate itself.